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2.2 Spur gears
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2.2 SPUR GEARS

The most common toothed gears are the so-called spur gears, which have their teeth radially on the external (or internal) surface of the gear. In particular, the spur gears are used to transmit the rotation motion between two parallel axes (or shafts).

The data of the maximum torque shown in the tables of the technical data sheet are the result of a cross between the theoretical calculations and the experimental data obtained in the laboratory.

Theoretical calculations are based on the Lewis formula. According to this formula the tooth (Fig.2), considered as a shelf wedged on the gear, does not yield under the action of the force (F) (considered as static force) transmitted by the gears. This theory is based on the following hypotheses:

  • the stress of the total force (F) on the tooth is considered as applied to the tip of the tooth itself
  • the radial component of the force (Fr) which determines a compression stress on the tooth is considered negligible; it follows that the component of force (F) that determines the bending of the tooth is the only one considered and, for simplicity, will have the same value as the tangential force (Fr) on the pitch circle
  • for the calculation the most unfavorable situation is taken into consideration, with only one pair of teeth engaged

The force (Ft) is then correlated to the torque (M) by means of the pitch diameter:

The experimental data has been obtained by laboratory tests and checked with software taking into consideration the VDI 2736 guideline for the design of technopolymer toothed gears. The tests have been carried out in continuous operation and at a speed of 100-150 rpm without any lubrication, to test the most severe conditions.

The torques in the tables of the technical data sheets provide a rough information and cannot be considered valid for every possible application. The operating conditions (rpm, working temperatures, coupling with transmission elements made of different materials, lubricated or dry conditions, service factor, etc.) strongly influence the performance.

The design engineer must take into account the actual conditions of use, different from those of the laboratory.